Lubricant formulations for diesel engine applications, particularly medium speed diesel (MSD) engines such as used in railroad applications in severe duty service have to be changed routinely over a relatively short period of time. A typical MSD engine oil is commonly changed after about 184 in service days. It is believed that small amount of oil soluble copper corrosion products, typically from the engine oil coolers, may accumulate in the lubricant to the point that the copper causes catalytic oxidation of the oil which causes an increase in the lead in the lubricant. Accordingly, to prevent the oxidation of oil components, oxidation inhibitors are added to the lubricant. However, conventional oxidation inhibitors are effective in the presence of copper for only a short period of time. Hence, the lubricant service life is significantly shortened. There is a need therefore for a lubricant formulation that is effective to increase the service life (drain interval) of the lubricant even in the presence of copper corrosion products so that oxidation of the oil and corrosion of lead components in the engine is minimized.
With regard to the foregoing, embodiments of the disclosure provide a method for maintaining a total base number (TBN) above about 3 mg KOH/g for a lubricant for a diesel engine for an extended period of time of greater than about 184 days of use of the lubricant in service in the engine. The method includes supplying as the lubricant, a lubricant composition comprising an alkali or alkaline earth metal sulfonate detergent having a TBN ranging from about 20 to about 100 mg KOH/g and an alkali or alkaline earth metal phenate detergent having a TBN ranging from about 200 to about 350 mg KOH/g and an antioxidant selected from the group consisting essentially of hindered phenolic derivatives of C3 to C6 acids and C7 to C9 esters of hindered phenolic derivatives of C3 to C6 acids. The phenate detergent is present in an amount that is greater than 15 percent by weight of the total weight of detergents in the lubricant composition and the lubricant composition is substantially devoid of zinc dialkydithiophosphate compounds and has a sulfated ash content of less than 1.0 wt. %. The engine is operated on the lubricant composition for an extended period of time between lubricant drain intervals.
An advantage of the disclosed embodiments is that drain intervals for lubricants used in diesel engines, such as medium speed diesel engines used for railroad applications, may be significantly extended even in the presence of copper corrosion products that may catalyze oxidation of the lubrication oil and damage engine components. For example, a 50% increase in the drain interval for such lubricant, even in the presence of copper ion corrosion products, is very desirable and may significantly reduce oil and maintenance costs for operating such engines. Another advantage of the disclosed embodiments is the unexpected increase in total base number (TBN) retention of the lubricant composition compared to conventional lubricant compositions. Other benefits and advantages of the compositions and methods described herein may be found in the following detailed description and examples.